I've posted this a couple of times in the past 2 weeks. Americans can buy BTC, ETH and LTC on
Coinbase. Click to earn a $10 (in BTC) bonus.
https://www.coinbase.com/join/569d36c9dca944488d000409
For the first few buys, I had a $100 weekly max using my Credit Card. After a couple of buys, it is now $750. ACH/Direct Deposit bank accounts is like $10,000 max.